This invention relates generally to mounting electrical components on printed circuit boards and particularly relates to mounting the electrical components substantially coplanar with the median plane or thickness of the board using solder reflow techniques. This obtains a higher density of electrical components and circuit board than mounting the components on the surfaces of the circuit board.
Printed circuit card assemblies long have mounted components on one side or the other of the printed circuit board. Early, the components terminated in leads that passed through plated holes in the circuit board and that were crimped to fasten the components to the board. The board and components then passed through a solder bath or solder wave to finish the desired soldered electrical connections. Recently integrated circuit packages with gull wing leads or J-leads and lead-less but capped resistors, capacitors and inductors became mounted to circuit boards with surface mount technology. A solder paste is printed on the bonding pads formed on the circuit board. Mechanical pickers place the electrical components on the pads with the paste providing some adhesion of the components to the pads. The board and carried components then pass through a solder reflow oven or the like to melt the solder paste, with the surface tension of the molten solder holding the components in place on the pads. Afterwards, the solder solidifies mechanically to fastens the components to the board and electrically to connect the components in the circuits carried on the board.
Surface mount technology attains a greater component density than the earlier technology; it mounts more components on a certain area of printed circuit board through eliminating many plated through holes and eliminating or reducing the size and configuration of component leads. Surface mount technology achieves even greater density of components on the board by mounting such as decoupling capacitors under integrated circuit packages or carriers of such as dynamic random access memories (DRAMs). Further, components can be surface mounted on both sides of the printed circuit board.